It is conventional that gas sensors which are sensitive for one component of exhaust gas should be arranged in a position as close to the internal combustion engine as possible in the exhaust ducts of internal combustion engines to determine and regulate (lambda regulation) an air/fuel ratio of the internal combustion engine as a function of a signal supplied by the gas sensor. Such sensors are usually lambda probes which measure the oxygen concentration of the exhaust gas.
It is also conventional that catalytic converters which convert one or more pollutant components of the exhaust gas into less environmentally critical compounds may be integrated into the exhaust duct downstream from the gas sensor. Three-way catalytic converters promote oxidation of unburned hydrocarbons (HC) and carbon monoxide (CO) to carbon dioxide (CO2) and water (H2O), while at the same time supporting the reduction of nitrogen oxides (NOx) to nitrogen (N2). Almost complete conversion of pollutants is achievable only at a stoichiometric exhaust gas atmosphere of λ=1. A growing environmental awareness in combination with increasingly more stringent statutory emission limits necessitates extremely accurate lambda regulation and emission monitoring.
One problem with conventional exhaust systems is due to a catalyst-specific temperature threshold below which the catalytic converter has inadequate conversion activity or none at all. This minimum conversion temperature for light-off is typically approximately 350° C. This temperature is not reached in certain internal combustion engine operation situations, in particular for a certain period of time after a cold start, so that during these phases, the exhaust gas is emitted with virtually no cleaning. There are conventional internal combustion engine measures, e.g., late ignition, to make it possible to nevertheless achieve catalytic converter heating in such situations, and there are also non-internal combustion engine measures such as catalytic converter heating devices. However, these measures are always associated with increased fuel consumption and in some cases with a much greater complexity in terms of equipment.
Therefore, it is an object of the present invention to provide an exhaust system and an exhaust emission control method which may guarantee rapid catalytic converter heating with a simple and inexpensive arrangement and make it possible to achieve low pollution emission levels.